The present invention relates to a color liquid crystal display device in which a first display cell which has an optical shutter effect is disposed on a second liquid crystal cell which has a multi-color polarizing plate.
Display devices employing liquid crystals have advantages concerning low power consumption, reduced thickness and long life, and are used in a very wide range of applications including digital timepieces, pocket calculators, measuring instruments as well as, in recent years, instruments for automotive applications, liquid crystal TVs, and the like. Accompanying such a wide range of applications, demands have increased year after year for the provision of color display devices, and a variety of systems have already been devised. From the standpoint of practical applications, however, each have merits and demerits.
Two typical examples will be described below briefly.
First is exemplified a system which employs color filters and a twisted nematic liquid crystal cell (hereinafter referred to as TN liquid crystal cell). FIG. 1 is a schematic section thereof, in which a pair of polarizing plates 1, 2 are so arranged that their polarizing axes are oriented in the same direction, and a TN liquid crystal cell is inserted therebetween, the TN liquid crystal cell being obtained by twist-orienting a nematic liquid crystal composition. Color filters 5 of red (R), green (G) and blue (B), which are the three primary colors for light, are provided on transparent electrodes 6 disposed on one substrate 4 of the liquid crystal cell.
When no voltage is applied thereto, light is not permitted to pass through, and black is displayed. When a voltage is applied, however, the filter to which the voltage is applied displays its color. That is, if the filter has a fine pattern, the device theoretically works in the same manner as a color CRT, which makes it possible to realize a full color spectrum by the method of additive color mixing. According to this system, however, light is absorbed by the filter even when it is white, and the display has a very small transmission factor. For example, when red is to be displayed, the other picture elements of green (G) and blue (B) appear to be black. With a reflective type of device, therefore, the display produced has a dark reddish color. Therefore, a satisfactory color display is not realized unless the device is constructed to be of a transmission type which uses back light of a very high brightness. This cancels the advantage of low power consumption which is the greatest merit of a liquid crystal display device, and greatly restricts the range of applications.
Next is exemplified a system which employs a multi-color polarizing plate and a TN liquid crystal cell, FIG. 2 is a schematic section thereof. The multi-color polarizing plate consists of a plurality of color polarizing plates arranged within one polarizing plate. For instance, the multi-color polarizing plate has red (R), green (G) ahd blue (B) polarizing plates those are arranged in the form of stripes, as designated by 7 in FIG. 2. Transparent electrodes 6 are also provided so as to correspond to the stripes on the multi-color polarizing plate. If the polarizing axes of the polarizing plate 1 and the multi-color polarizing plate 7 are at right angles with each other, light is permitted to pass through when no voltage is applied, and a white display is produced. When a voltage is applied, on the other hand, light is absorbed by the color polarizing plate at the portion to which the voltage is applied, and the color of this color polarizing plate is displayed. If the stripes of the color polarizing plate have a small pitch, additive color mixing can be realized. For instance, a yellow display can be produced by applying voltages to the red (R) and green (G) polarizing plates. However, when, for example, a red display is produced, the other green and blue picture elements will produce white. Therefore, a dark reddish display is not produced even with a reflective type of device employing color filters. In this case, however, a display or a pale reddish color with a small saturation is produced. That is, although the display with a higher transmission factor than that of the system employing color filters is produced, it is difficult to obtain a sufficiently high contrast.